Ink jet printers include a drop ejection device and a supply of printing fluid for providing printing fluid such as ink to the ejection device. In the case of thermal ink jet printing, the drop ejection device is typically referred to as a printhead. Printing is accomplished by the selective actuation of the printhead as the printhead is moved relative to a print media. One common type of previously used ink jet printer uses a replaceable print cartridge which contains a printhead and a supply of ink contained within the print cartridge. This type of print cartridge is not intended to be refillable. When the initial supply of ink is depleted, the print cartridge is disposed of and a new print cartridge is installed.
Another type of ink jet printer makes use of an ink reservoir that is separately replaceable from the printhead. The replaceable reservoir can be positioned on a scanning carriage with the printhead or positioned off the scanning carriage. In the case where the ink cartridge is mounted off carriage, the ink cartridge can be continuously in fluid communication with the printhead such as connected by a flexible conduit or intermittently connected by positioning the carriage at a refilling station. The use of a replaceable ink container allows for the replacement of the ink container separate from the printhead allowing the use of the printhead until end of printhead life reducing the cost per page of printing.
Regardless of ink jet printer configuration, it is important that the system have an accurate means of indicating low ink levels to avoid the nuisance and resource waste of exhausting one of the ink supplies in the middle of a printing job. Moreover, it is important that the system stop printing when the ink cartridge is nearly empty. Allowing the ink jet printer cartridge to reach the state of complete ink exhaustion can result in operation of the thermal printhead without ink which can result in catastrophic damage and failure of the printhead.
One such ink level detector is disclosed in U.S. Pat. 5,616,929 to Hara where an optical ink detection section formed from a light transmitting material has an inclined interface and is formed integrally with the ink tank. Incident light from a light source is refracted when ink is present in the ink tank at the interface to make refraction light and does not emerge to a visual observation portion. When no ink is present, the incident light is totally reflected by the interface to make total reflection light, which can be visually observed. Hara teaches the use of a system that detects absence or presence of liquid ink based on the presence or absence of reflected light at a light receiving element. In order to detect ink levels at other discrete locations on the ink tank, a light source, a light receiving element, and a light interface is required at each desired ink detection section. This would be costly as well as space prohibitive in the inexpensive, small printers of the current technology.
There are clear advantages to having the ability to detect ink levels at numerous positions on the ink tank. For example, with large format printers which use a considerable amount of ink for covering large printing surfaces, the ability to compare ink requirements with ink remaining in the ink container prior to printing would be invaluable. In addition, providing more comprehensive feedback to the user of ink usage allows the user to better anticipate when the ink containers will require replacing.